明渠紊流测量中的梯度效应

发布时间：2018-05-24 08:41

本文选题：PTV + 测量偏差　；参考：《清华大学》2015年博士论文

【摘要】：应用PTV测速技术可以获得离散的瞬时速度数据,在数据后处理过程中,需在有限大小的采样窗口内对采样数据进行统计平均。由于流速梯度、浓度梯度和相间速度差的存在,在采样窗口内的平均过程将产生统计误差(或称测量偏差)。本文在前人研究工作的基础上,以明渠水沙两相流为研究对象,对上述问题进行了较为全面的研究。垂线流速分布选用对数型公式和指数型公式并进行对比分析,浓度分布选用Rouse公式;经理论分析推导出了平均速度、紊动强度测量偏差解析表达式,通过对表达式的理论分析和数值计算,分析测量偏差的大小及其随影响因素的变化规律。由于流速梯度的存在,在采样窗口内测得的平均流速值与采样窗口中心点处的真实值相比偏小,而估算的紊动强度值要高于真实的紊动强度值,该结论与选用的两种具体流速分布型式无关。由流速梯度引起的测量偏差与流速分布型式、采样窗口尺寸?h、窗口中心位置my有关,测量偏差等值线图呈单调递增函数曲线。若采样窗口的尺寸不变,测量偏差在明渠流的上部区域相对要小,在靠近床面的下部区域则相对要大;相对于对数型公式而言,指数型公式得到的偏差略大。实验室和野外实测资料分析表明,平均速度测量偏差在整个水深范围内均不大,常可忽略不计,但紊动强度的测量误差相对较大,不可忽略。浓度梯度的存在使得示踪颗粒在采样窗口内分布不均,当同时存在流速梯度时将引起测量偏差,偏差大小与采样窗口尺寸、窗口中心位置、流速分布型式以及浓度分布的不均匀程度(颗粒悬浮指标Z)四个因素有关。平均速度的实测值小于真实值,而紊动强度实测值大于真实值,且与清水水流相比,浓度梯度效应使得紊动强度测量偏差进一步增大。平均速度、紊动强度测量偏差等值线图自床面向水面并非单调函数曲线,拐点出现在相对位置y/h?0.3?0.5处。采用传统“浑水流”模式得到的混合物的平均速度与PTV测量得到的固、液相各自平均速度偏差的正负取决于相间速度差?U的正负;在特定水流条件下,固相颗粒浓度越高,混合物的平均速度就偏向于颗粒的速度分布,反之亦然;理论分析和实验结果均表明,混合物的紊动强度有可能小于水流的紊动强度,也有可能大于水流的紊动强度。通过对固、液相各自实验数据进行简单算术平均来确定浑水运动参量的处理方式是不当的。
[Abstract]:The application of PTV speed measurement technology can obtain discrete instantaneous velocity data. In the process of data processing, the sampling data must be statistically averaged within a limited sampling window. The statistical error (or measurement deviation) will be generated in the average process in the sampling window due to the existence of the velocity gradient, the concentration gradient and the interphase velocity difference. On the basis of the previous research work, the water and sand two phase flow in the open channel are studied in a more comprehensive way. The vertical flow velocity distribution selects the logarithmic formula and the exponential formula and carries on the comparative analysis. The concentration distribution is selected by the Rouse formula. The mean velocity and the turbulence intensity measurement deviation are derived by theoretical analysis. Through the theoretical analysis and numerical calculation of the expression, the size of the measurement deviation and the variation law of the influence factors are analyzed. The average velocity measured in the sampling window is smaller than the true value at the center of the sampling window because of the existence of the velocity gradient, and the estimated turbulence intensity is higher than the real turbulence intensity. Degree value, the conclusion is independent of the two specific velocity distribution patterns selected. The measurement deviation caused by the velocity gradient is related to the pattern of the velocity distribution, the size of the sampling window, h, the center position of the window, my, and the monotone increasing function curve of the measurement deviation contour map. If the size of the sampling window is unchanged, the measurement deviation is relative to the upper region of the open channel flow. To be small, the lower region near the bed is relatively large; the exponential formula has a slightly larger deviation than the logarithmic formula. The analysis of the laboratory and field measured data shows that the average velocity measurement deviation is not large in the whole water depth range, often negligible, but the measurement error of the turbulence intensity is relatively large and can not be ignored. The presence of the concentration gradient makes the tracer particles unevenly distributed in the sampling window. When the velocity gradient exists at the same time, the measurement deviation will be caused. The magnitude of the deviation is related to the size of the sampling window, the center position of the window, the pattern of the velocity distribution and the inhomogeneous degree of the concentration distribution (Z). The measured value of the average velocity is less than that of the concentration gradient. True value, and the measured value of the turbulence intensity is greater than the true value, and compared with the clear water flow, the concentration gradient effect makes the turbulence intensity measurement deviation further increase. The average velocity, the turbulence intensity measurement deviation contour map from the bed surface is not monotonous function curve, the inflection point appears in the relative position y/h? 0.3? 0.5. The traditional "muddy water flow" is used. The mean velocity of the mixture obtained by the model is determined by the mean velocity measured by PTV and the positive and negative of the average velocity deviation of the liquid phase depends on the difference between the phase velocity and the positive or negative of the U. The higher the concentration of the solid particles is, the average velocity of the mixture is biased toward the velocity distribution of the particles, and vice versa; both theoretical analysis and experimental results show that The turbulence intensity of the mixture may be less than the turbulence intensity of the flow, and it may be greater than the turbulence intensity of the flow. It is not appropriate to determine the motion parameters of the muddy water by the simple arithmetic average of the experimental data of the liquid phase.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TV133

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